FIG. 11 is a circuit diagram for a cooling medium showing a refrigerator as a conventional type of refrigerating/air-conditioning apparatus disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 5-157379. In the figure, designated at the reference numeral 1 is a compressor, at 2 a heat-source-side heat exchanger, at 3 a decompressor for a cooling medium as a capillary, and at 4 a user-side heat exchanger, and those components are serially connected through a piping and constitute a refrigeration cycle. The reference numeral 5 is a heat exchanger for heat-exchanging between the decompressor 3 and a suction pipe for the compressor 1.
As a cooling medium in this refrigerator, for example, HFC134a is used, and as a refrigerating machine oil therein, for example, alkylbenzene-based oil having no or extremely low mutual solubility to HFC134a is used.
Next description is made for operations by using a pressure-enthalpy diagram. In the refrigerator having the configuration, vapor from a cooling medium (point A in the figure) under a high temperature and a high pressure compressed by the compressor 1 is condensed by the heat-source-side heat exchanger 2 to become a vapor-liquid two-phase cooling medium (point B in the figure) having dryness of around 0.1 as a quantity ratio of a liquid phase cooling medium in the vapor-liquid two-phase cooling medium, and is decompressed by the decompressor 3 for the cooling medium to flow into the user-side heat exchanger 4 as a vapor-liquid two-phase cooling medium under a low temperature and a low pressure (point C in the figure). Further, this cooling medium evaporates in the user-side heat exchanger 4, and returns to the compressor 1 through the heat exchanger 5 to be compressed again. The refrigerating machine oil discharged with the cooling medium from the compressor 1 circulates through a cooling medium circuit together with a vapor phase cooling medium as well as a liquid phase cooling medium, and returns to the compressor 1.
In this type of refrigerating/air-conditioning apparatus, alkylbenzene-based oil having no or extremely low mutual solubility to a cooling medium but being excellent in lubricity and abrasion resistance against a sliding section in the compressor 1 is used as refrigerating machine oil, so that it is possible to obtain a refrigerating/air-conditioning apparatus with high-reliability by securely returning the refrigerating machine oil to the compressor.
As described above, in the conventional type of refrigerating/air-conditioning apparatus, operational conditions and loading conditions are substantially constant, and when a flow rate of the cooling medium circulating a cooling medium circuit is sufficiently insured, refrigerating machine oil circulates with a cooling medium to flow back to the compressor without occurrence of excessive stagnation in a pipe and a capillary in the cooling medium circuit. Also, in the conventional type of refrigerating/air-conditioning apparatus, a state of a cooling medium at an outlet of the heat-source-side heat exchanger 2 is a vapor-liquid two-phase cooling medium, so that there is no pipe for a liquid with only liquid phase cooling medium flowing therethrough, and for this reason consideration on the stagnation of the refrigerating machine oil in the pipe for a liquid is not necessary.
As the conventional type of refrigerating/air-conditioning apparatus has the configuration as described above, if the operational conditions and loading conditions are largely changed, a flow rate of a cooling medium is reduced, or an oil rate in the refrigerating machine oil discharged from the compressor 1 increases, a life of the apparatus is largely reduced because an oil rate stagnating in the cooling medium increases, an oil rate flowing back to the compressor 1 decreases, and bad lubricity or the like occurs due to shortage of the refrigerating machine oil in the compressor 1.
Also, if a large amount of refrigerating machine oil is stagnated in a heat transfer pipe for the heat-source-side heat exchanger 2 as well as for the user-side heat exchanger 4, performance of heat transfer is reduced, pressure loss is increased, and energy efficiency of the refrigerating/air-conditioning apparatus is reduced or similar problems may occur.
Further, when refrigerating machine oil having no or extremely low mutual solubility to a cooling medium is used for a refrigerating/air-conditioning apparatus in which a pipe for a liquid such as an outlet section of the heat-source-side heat exchanger 2 with only a liquid phase cooling medium flowing therethrough exists over a long distance, an amount of the refrigerating machine oil stagnating in this pipe increases, an oil rate flowing back to the compressor 1 decreases, and bad lubricity or the like occurs due to shortage of the refrigerating machine oil in the compressor 1.